There are some Nixpkgs specific issues that we could fix but due to
changes in Git it currently isn't possible to source git-completion.bash
from ZSH (at least not how tig-completion.bash expects it).
Upstream issue: https://github.com/jonas/tig/issues/940
For the meantime it seems best to simply not install it anymore so that
the fallback implementation from ZSH can be used (more inaccurate as the
git-log completion is reused but it is helpful to complete remotes,
branches, tags, etc. and doesn't emit an error to the console).
e9c36545a7